These experiments will systematically investigate the thermal sensitivity, the ability to develop thermal tolerance and the ability to induce heat shock protein synthesis in early mammalian zygotes in vitro. The experiments also will systematically investigate the thermal sensitivity and the ability to induce thermal tolerance in mammalian germ cells, as assayed by the frequency of dominant lethal mutations expressed in mammalian zygotes in vitro. Our long term objective is to apply the unique aspects afforded by heating the mammalian zygone and/or the germ cell to elucidate the mechanisms involved in thermal killing, the induction of thermal tolerance and the enhanced synthesis of heat shock proteins in cells. Mouse zygotes in different developmental states will be heated for various lengths of exposure times at different temperatures (39-43 degrees C). Survival will be measured by their ability to complete cleavage and to successfully form a blastocyst. Heating and embryo growth will be in vitro. In split dose experiments, heat will be given in 2 fractions separated by 4, 8, and 24 hours. The ability to develop thermotolerance and to induce enhanced synthesis of heat shock proteins will be determined. Thermal damage to the germ cell is reflected by the induction of dominant lethal mutations. The frequency of successful development in vitro of zygotes sired by a heated parent will be used as the index of heat-induced damage. This will be tested at various temperatures, durations of the treatment and at different germ cell stages. These experiments will provide information on the thermal response of mammalian zygotes and germ cells.